feat(dashboard): channel keyword search — closes #497

[kitWarse]

Closes #497

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Summary

Implements issue #497 — Allow users to search available channels using keywords.

Changes

dashboard/src/pages/NotificationPreferencesPage.tsx

• Added channelSearch state and channelSearchRef for focus management after clear
• Added ilteredChannels useMemo — case-insensitive filter on channelDefinitions by keyword
• Search input rendered above the delivery matrix with proper �ria-label + �ria-controls linkage
• Both column headers and toggle cells now iterate ilteredChannels (not the static array)
• Clear (✕) button returns focus to the input after resetting
• No-results empty state with
  ole=status + �ria-live=polite + inline Clear search button
• gridTemplateColumns inline style shrinks the matrix grid to match visible columns

dashboard/src/index.css

• .channel-search / .channel-search__label / .channel-search__input — flex layout + focus ring
• .channel-search__clear / .channel-search__clear-inline — clear button styles
• .notification-preferences__no-results — centred dashed-border empty state

Acceptance Criteria

Criterion	Status
Search filters results	✅ filteredChannels updates synchronously on every keystroke
No-results state is displayed	✅ Dashed-border empty state with clear-search action
Search performs efficiently	✅ Synchronous O(n) filter on a 4-item static list — zero async overhead

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